1. Field of the Invention
The present invention relates to a braking force control apparatus for a vehicle, which is configured to brake a wheel by using both of a regenerative brake using an electric power generation operation of a motor and a mechanical brake using application of a mechanical resistance.
2. Description of the Related Art
Hitherto, there has been known a vehicle configured to use a motor to generate a driving force and a braking force (referred to as braking/driving force) on a wheel. In this type of vehicle, both of a friction braking force and a regenerative braking force are used to brake the wheel. The friction braking force is generated by using a hydraulic pressure to bring brake pads into contact with a disc (or a drum), and the regenerative braking force is generated by recovering electric power, which is generated by using the motor for driving the wheel to act as an electric power generator, to a battery. An apparatus proposed in Japanese Patent Application Laid-open No. Hei 05-161211 is configured to prioritize the regenerative braking over the friction braking in order to exert an energy recovery effect through the regenerative braking.
The braking/driving force on the wheel is converted into a force in a vertical direction of a vehicle body by a suspension. Thus, in a vehicle including a motor as a braking/driving power source for a wheel, a motion state of the vehicle can be controlled by controlling the braking/driving force of each of the motors. In a vehicle of an in-wheel motor type in which a motor is arranged inside or near each of the front/rear wheels, vehicle motions such as a roll motion, a yaw motion, a pitch motion, and a heave motion of the vehicle body can be controlled by applying power running control or regenerative control to each of the motors. Moreover, also in a vehicle in which the left and right wheels of the front wheels or the rear wheels are independently driven by motors, the roll motion, the yaw motion, the pitch motion, and the like can be controlled by controlling the braking/driving forces of the motors. Moreover, also in a vehicle in which the left and right wheels of the front wheels or the rear wheels are driven by a common motor, the pitch motion can be controlled by controlling the braking/driving force of the motor.
When the vehicle motion control is carried out, and an operation by a driver is an accelerator pedal operation, a target braking/driving force generated on the wheel by a motor can be calculated as a sum of a driver-requested driving force based on an accelerator operation amount and a control braking/driving force required for carrying out the vehicle motion control. On the other hand, when the operation by the driver is a brake pedal operation, a driver-requested braking force based on a brake operation amount is set, and the driver-requested braking force is distributed to a requested friction braking force and a requested regenerative braking force. The target braking/driving force to be generated by the motor on the wheel can be calculated as a sum of the requested regenerative braking force and the control braking/driving force required for the vehicle motion control. Even during the brake operation, the vehicle motion control can be carried out by controlling an operation of the motor based on the target braking/driving force.
There is a limit to regeneration performance. Therefore, during the brake pedal operation, the control braking/driving force that can be generated by the motor, and can be used for the vehicle motion control is limited to a range acquired by excluding the requested regenerative braking force required for decelerating the vehicle from the maximum braking force that can be generated by the motor. Thus, as the requested regenerative braking force increases, a margin of the control braking/driving force that can be used for the vehicle motion control decreases. Under the state in which the margin of the control braking/driving force is decreased, the target driving force of the motor is liable to reach a performance limit, and satisfactory vehicle motion control may not be carried out.